1. Field of the Invention
The subject invention relates generally to electrical circuit fabrication and, more particularly, to a method and apparatus for measuring the quality of interconnection made by a bonding machine as it operates.
2. Description of Related Art
In the prior art, wirebonders are known for establishing wire interconnection to electrical contacts, for example, between an integrated circuit (IC) pad and substrate gold. One such commercially available wirebonder is the Model HMC 2460 thermosonic wirebonder available from Hughes Aircraft Company. Such a wirebonder includes an ultrasonic transducer, an ultrasonic feedhorn, and a bond tool or capillary. Mechanical force and ultrasonic energy are applied to the capillary to create a wirebond between a bond surface and a gold ball having a diameter of, e.g., 0.7 to 2.0 mils.
In operation, the wirebonder applies heat to the bond site via a heated stage located underneath the bond surface. Force is applied by pressing the capillary down toward the bond surface. An ultrasonic signal is generated by the generator and converted to a mechanical ultrasonic frequency vibration of the feedhorn by the transducer. The capillary transmits the ultrasonic energy and downward force, effectively "scrubbing" the gold ball against the bond surface. The combination of heat, force, and ultrasonic vibration causes bonding between the bond surface and the gold ball.
Wirebonds formed by wirebonding apparatus have typically been required to go through laborious and expensive testing. For example, current military requirements specify internal visual inspection for determining the integrity of wirebonds and other microcircuit interconnections. Visual inspection of wirebonds has become a costly task. Production operators may review hundreds of thousands of initial build wirebonds and rework build wirebonds in a normal week. Given such facts, the industry has been moving towards automated methods for testing and verifying the integrity of interconnections such as those made by wirebonders. One such approach has been that referred to as bond signature analysis, such as the method disclosed in the copending U.S. Application of Owen E. Gibson et al. for a Bond Signature Analyzer, assigned to Hughes Aircraft Company, wherein an electrical signal indicative of bond quality is sampled and analyzed to determine bond quality. While useful, bond signature analysis has the disadvantage of being relatively complex and expensive to implement.